Tracing sediment dynamics and sources in eroding rills with rare earth elements

Eroding rills evolve morphologically in time and space. Most current studies on rill erosion use spatially averaged soil erosion data, providing little information on soil erosion dynamics. A method is proposed to use rare earth elements (REEs) to trace sediment distribution in eroding rills. Laboratory flume simulation experiments were conducted at three flow rates (2, 4 and 8 litres minute^?1) and five slope gradients (5, 10, 15, 20 and 25°) with three replicates of each treatment. The rills, of 8 m length, were subdivided into 10 equal segments of 0.8 m length and 0.1 m width, with a different REE applied to each segment. We derived computational formulae for estimating the distribution of eroded amounts along the rills. The actual erosion distribution along rills was then estimated with the data from the experiments. The precision of the REEs for tracing rill erosion was analysed. The results showed that sediment concentration increased with rill length, but the increased rate (the slope of the curve) flattened gradually. Sediment yields increased with slope gradients and flow rates, but the slope gradients had a greater effect on sediment concentration than flow rates, and greater flow rates caused more rill erosion and soil loss under the same slope gradient. The results also demonstrated the feasibility of using REEs to trace the dynamic processes of rill erosion.